Magnetic memories, particularly magnetic random access memories (MRAMs), have drawn increasing interest due to their potential for high read/write speed, excellent endurance, non-volatility and low power consumption during operation. An MRAM can store information utilizing magnetic materials as an information recording medium. One type of MRAM is a spin transfer torque random access memory (STT-RAM). STT-RAM utilizes magnetic junctions written at least in part by a current driven through the magnetic junction. A spin polarized current driven through the magnetic junction exerts a spin torque on the magnetic moments in the magnetic junction. As a result, layer(s) having magnetic moments that are responsive to the spin torque may be switched to a desired state.
For example, FIG. 1 depicts a conventional magnetic tunneling junction (MTJ) 10 as it may be used in a conventional STT-RAM. The conventional MTJ 10 typically uses conventional seed layer(s) 12 and includes a conventional antiferromagnetic (AFM) layer 14, a conventional pinned layer 16, a conventional tunneling barrier layer 18, a conventional free layer 20, and a conventional capping layer 22. The conventional seed layer(s) 12 are typically utilized to aid in the growth of subsequent layers, such as the AFM layer 14, having a desired crystal structure. The conventional tunneling barrier layer 18 is nonmagnetic and is, for example, a thin insulator such as MgO.
The conventional pinned layer 16 and the conventional free layer 20 are magnetic. The magnetization 17 of the conventional pinned layer 16 is fixed, or pinned, in a particular direction, typically by an exchange-bias interaction with the AFM layer 14. Although depicted as a simple (single) layer, the conventional pinned layer 16 may include multiple layers. For example, the conventional pinned layer 16 may be a synthetic antiferromagnetic (SAF) layer including magnetic layers antiferromagnetically coupled through thin conductive layers, such as Ru. In such a SAF, multiple magnetic layers interleaved with a thin layer of Ru may be used. In another embodiment, the coupling across the Ru layers can be ferromagnetic. Further, other versions of the conventional MTJ 10 might include an additional pinned layer (not shown) separated from the free layer 20 by an additional nonmagnetic barrier or conductive layer (not shown).
The conventional free layer 20 has a changeable magnetization 21. Although depicted as a simple layer, the conventional free layer 20 may also include multiple layers. For example, the conventional free layer 20 may be a synthetic layer including magnetic layers antiferromagnetically or ferromagnetically coupled through thin conductive layers, such as Ru. Although shown as in-plane, the magnetization 21 of the conventional free layer 20 may have a perpendicular anisotropy. Thus, the pinned layer 16 and free layer 20 may have their magnetizations 17 and 21, respectively oriented perpendicular to the plane of the layers.
To switch the magnetization 21 of the conventional free layer 20, a current is driven perpendicular to plane (in the z-direction). When a sufficient current is driven from the conventional free layer 20 toward the conventional AFM layer 14, the magnetization 21 of the conventional free layer 20 may switch to be parallel to the magnetization 17 of the conventional pinned layer 16. When a sufficient current is driven from the conventional AFM layer 14 toward the conventional free layer 20, the magnetization 21 of the free layer may switch to be antiparallel to that of the pinned layer 16. The differences in magnetic configurations correspond to different magnetoresistances and thus different logical states (e.g. a logical “0” and a logical “1”) of the conventional MTJ 10.
Although the conventional MTJ 10 may be written using spin transfer and used in an STT-RAM, there are drawbacks. Magnetic memories are desired to have a high density. Use of the conventional MTJ 10 as a memory element in STT-RAM may not provide the desired density given the areal requirements of the conventional MTJ 10 and associated circuitry. FIG. 2 depicts a conventional memory cell 50 that is a solution for providing a higher density memory using the conventional MTJ 10. The conventional memory cell 50 stacks multiple conventional MTJs 10. For clarity, the conventional MTJs are labeled 10′, 10″, and 10′″. Each MTJ 10′/10″/10′″ is separated by a nonmagnetic, metallic spacer 52. The conventional cell 50 might also include contacts (not shown) as well as a selection device (not shown), such as a transistor. In operation, the conventional MTJs 10′, 10″, and 10′″ are switched using different switching currents. Each conventional MTJ 10′, 10″, and 10′″ also has two stable states. Thus, the combination of the MTJs 10′, 10″, and 10′″ in the memory cell 50 may store three bits (e.g. logical states 000, 001, 010, 011, 100, 101, 110, 111).
Although the conventional memory cell 50 functions, there are drawbacks. Fabrication of the conventional memory cell 50 may be challenging. Each conventional MTJ 10′, 10″, and 10′″ is typically on the order of thirty-sixty nanometers thick. Further, each conventional metallic spacer 52 is typically ten to fifteen nanometers thick. Thus, the total stack height of the memory cell 50 may be on the order of one hundred nanometers or more. In contrast, the width of the stack for the memory cell 50 is on the order of tens of nanometers. Fabrication of a structure having such a high aspect ratio may be challenging and yield may be poor.
Accordingly, what is needed is a method and system that may improve the density of the spin transfer torque based memories without significantly complicating processing or reducing yield. The method and system described herein address such a need.